Britain on Monday urged the United States to honour free-trade commitments made at the G7 summit even after US President Donald Trump rejected a joint communique in a bitter spat over tariffs.

"The communique was agreed by all the parties who attended the G7 and we fully intend to honour it," the spokesman said.

"We would hope that the US will similarly honour the commitments that they have made," he said, speaking before a parliamentary statement on the summit by Prime Minister Theresa May.

May told MPs that it had been "a difficult summit with, at times, some very candid discussions", adding that there had been "strong debate and disagreement on some issues".

In a pointed criticism of Trump's policy, May said that the most effective way of reaping the benefits of globalisation was through multilateral action, adding: "It cannot be done by taking unilateral action against your partners".

"As long-standing allies, we do not make progress by ignoring each others' concerns but rather by addressing them together," she said.

The summit in Canada was deeply divisive and ended with a renewed threat of trade war on Saturday after Trump rejected an attempt to write a consensus statement and bitterly insulted the Canadian host.

The joint communique that was thrashed out over two days of negotiations vowed that members would reform multilateral oversight of commerce through the World Trade Organization (WTO) and seek to cut tariffs.

"We commit to modernize the WTO to make it more fair as soon as possible. We strive to reduce tariff barriers, non-tariff barriers and subsidies," it said, reflecting the typical language of decades of G7 statements.

Trump had already said he would not hesitate to shut countries out of the US market if they retaliate to steep tariffs he has imposed on steel and aluminium imports.

The joint communique was published online before Trump tweeted his disapproval after comments made by Canadian Prime Minister Justin Trudeau on the new trade tariffs.